An information display device functioning as an information transfer path between machines and human has been rapidly developed based on an advanced manufacturing technology and electronic information industry. Particularly, a liquid crystal display (hereinafter, referred, to as LCD) using liquid crystal has been rapidly growing for the last ten years on the basis of a high-leveled semiconductor process technology and industrial infrastructures, and then nowadays acquires very wide application product groups and occupies the most powerfully its related market share.
The LCD is what is called a non-self-emissive type LCD which does not emit light by itself. Therefore, the LCD should necessarily include another light source and provide light to a display panel in order to display images. Many LCDs, which are now being provided to the market, display images by including Backlight Units (BLU) as light sources placed at a back side of the panel and by selectively transmitting or blocking light emitted from the backlight units. As such, the backlight unit being placed at the back side of the panel and providing light to the display device determines main factors of the LCD, for example, power consumption, image quality, thickness and price and so on.
The backlight unit can be configured in various shapes in accordance with a size of a finished product, use, desired brightness and the like. Here, a structure of the backlight unit to be used in a portable information display device includes a point light source and a line light source such as a Light Emitting Diode (LED) or a Cold Cathode Fluorescent Lamp (CCFL), a Light guide plate (LGP) uniformly emitting light emitted from the point light source and the line light source over the entire surface of the panel and changing into a surface light source, a prism sheet correcting an orientation angle and a spatial uniformity of the light emitted from the light guide plate, and a diffuser sheet and the like.
Here, the light guide plate changing the light coming from the lateral side thereof into a surface light source emits the light upward by means of a very small sized structure formed on the top surface or the bottom surface of the light guide plate. The shape of the structure plays an important role of determining the orientation angle and luminance of light emitted from the light guide plate. A light emitting structure of the light guide plate proposed in a conventional technology includes a micro lens, a micro prism, a straight line shaped projection and a curved line shaped projection, etc. However, the light emitted from such a structure has a very low light emitting efficiency, so that a ratio of luminance to power consumption is not high. The light emitted from such a structure usually does not proceed vertically, so that light path should be corrected through a separate prism sheet. Therefore, these are major causes of increasing the whole thickness and price of the display device and of reducing power efficiency of the display device.
FIG. 1 is a perspective view showing a structure of a backlight unit (BLU) according to a conventional technology. FIG. 2 is a cross sectional view showing the backlight unit according to the conventional technology. FIG. 3 is a perspective view showing a structure of a light guide plate being disposed on the backlight unit according to a conventional technology and including a lens-shaped projection on the top surface thereof. FIG. 4 is a perspective view showing a structure of a light guide plate being disposed on the backlight unit according to a conventional technology and including a pyramid-shaped projection on the top surface thereof.
As shown in FIGS. 1 and 2, a backlight unit 100 according to the conventional technology includes alight source 110, a light guide plate (LGP) 120 receiving light emitted from the light source 110 and propagating the light, a diffuser sheet 130 correcting the characteristic of the light emitted from the light guide plate, a prism sheet 140 and other optical functional sheets and protective films.
FIGS. 1 and 2, the operation of the backlight unit according to the conventional technology will be described. After light 111 emitted from the light source 110 is incident on one side of the light guide plate 120, the light is propagated without being emitted outward due to optical total internal reflection occurring at an interface between mediums having mutually different refractive indexes.
As shown in FIGS. 3 and 4, the light guide plate installed in the backlight unit according to the conventional technology includes a particular projection structure having a shape of a lens 123 or a pyramid 123′ on the top surface or bottom surface thereof. The projection structure causes the light to be emitted upward from the inside of the light guide plate 120.
Here, when the projection structures 123 and 123′ i.e., a light emitting pattern including a peculiarly shaped unevenness formed on a top surface 121 or a bottom surface 122 thereof are formed, a total internal reflection condition is not satisfied at a certain corresponding position. Therefore, the incident light may be emitted outward.
Since the light 112 emitted outward originally tends to emit toward the opposite side to that of the light source, the prism sheet 140 formed vertically and horizontally is required to make the direction of the light to be perpendicular to the surface of the display device.
In order to remove a pattern generated from the light emitting pattern formed on the top surface 121 or the bottom surface 122 of the light guide plate, a scattering sheet or a diffuser sheet 130 should be included.
As such, the backlight unit including the conventional light guide plate 120 is required to additionally include the multi-layered optical sheets 130 and 140. Therefore, there is a problem in that large losses are expected in terms of production cost, thickness and optical luminous efficiency.
FIGS. 5 to 8 are views of optical simulation results showing a light emission characteristic of the backlight unit light guide plate employing a conventional prism structure or lens structure. FIGS. 5 and 6 show angular luminance charts for the light emission characteristic of the backlight unit employing the conventional prism structure (referred to as a pyramid structure in FIG. 5) or lens structure. FIGS. 7 and 8 show spatial luminance charts of the backlight unit employing the conventional prism structure or lens structure.
As shown in FIGS. 5 and 6, since the light guide plate employing the conventional prism structure or lens structure has very low vertical light emitting efficiency, required necessarily is a separate optical sheet which corrects light having very low vertical light emitting efficiency to be perpendicular to the light guide plate.
As shown in FIGS. 7 and 8, the light guide plate employing the conventional prism structure or lens structure has generally low spatial luminance distribution.